[InstCombine] [NFC] Add tests for peeking through FP casts for sign-bit compares (PR36682)

Summary:
This pattern came up in PR36682:
https://bugs.llvm.org/show_bug.cgi?id=36682
https://godbolt.org/g/LhuD9A

Tests for proposed fix in D44367.

Looking at the IR pattern in question, as per [[ https://github.com/rutgers-apl/alive-nj | alive-nj ]], for all the type combinations i checked
(input: `i16`, `i32`, `i64`; intermediate: `half`/`i16`, `float`/`i32`, `double`/`i64`)
for the following `icmp` comparisons the `sitofp`+`bitcast` can be dropped:
* `eq 0`
* `ne 0`
* `slt 0`
* `sle 0`
* `sge 0`
* `sgt 0`
* `slt 1`
* `sge 1`
* `sle -1`
* `sgt -1`
I did not check vectors, but i'm guessing it's the same there.
{F5887419}

Thus all these cases are in the testcase (along with the vector variant with additional `undef` element in the middle).
There are no negative patterns here (unless alive-nj lied/is broken), all of these should be optimized.

Generated with {F5887551}

Reviewers: spatel, majnemer, efriedma, arsenm

Reviewed By: spatel

Subscribers: nlopes, wdng, llvm-commits

Differential Revision: https://reviews.llvm.org/D44390

llvm-svn: 327301

0 files changed, 0 insertions, 0 deletions